Earth boring bits for drilling oil and gas such as rotary conical bits or hammer bits incorporate carbide inserts as cutting elements. To improve their operational life, these inserts are preferably coated with an ultra hard material such as polycrystalline diamond. Typically, these coated inserts are not used throughout the bit. For example, diamond coated inserts are used to form the gage row 2 in roller cones 4 of a roller cone bit 3 (FIG. 11), or the gage row 1202 of a percussion bit 1203 (FIG. 12A). The inserts typically have a body consisting of a cylindrical grip from which extends a convex protrusion. The protrusion, for example, may be hemispherical, commonly referred to as a semi-round top (SRT), or may be conical, or chisel-shaped and may form a ridge that is skewed relative to the plane of intersection between the grip and the protrusion.
When installed in the gage area, for example, these inserts typically contact the earth formation away from their central axis 32 at a location 8 as can be seen with insert 5 on FIG. 11. The interfacial region between the diamond and the substrate is inherently weak in a diamond coated insert due to the thermal expansion mismatch of the diamond and carbide substrate materials. As a result, diamond coated inserts tend to fail by delamination of the diamond layer, either by cracks initiating along the interface and propagating outward, or by cracks initiating in the diamond layer surface and propagating catastrophically along the interface.
Two approaches have been used to address the delamination problem. One approach is to significantly increase the surface area of the interface through the use of corrugated or "non-planar" interfaces, which have the claimed effect of reorienting and reducing the interfacial stresses over the entire protrusion surface. The other approach uses transition layers, made of materials with thermal and elastic properties intermediate between the ultra hard material layer and the substrate, applied over the entire protrusion surface. These transition layers have the effect of reducing the residual stresses at the interface, thus, improving the resistance of the inserts to delamination. When the delamination problems, however, have been solved, new enhanced insert failure modes are introduced which are highly localized to the regions of the applied stress. These new failure modes involve complex combinations of three mechanisms. These mechanisms are wear of the PCD, surface initiated fatigue crack growth, and impact-initiated failure.
The wear mechanism occurs due to the relative sliding of the PCD relative to the earth formation, and its prominence as a failure mode is related to the abrasiveness of the formation as well as other factors such as formation hardness or strength, and the amount of relative sliding involved during contact with the formation.
The fatigue mechanism involves the progressive propagation of a surface crack, initiated on the PCD layer, into the material below the PCD layer until the crack length is sufficient for spalling or chipping.
The impact mechanism involves the sudden propagation of a surface crack or internal flaw initiated on the PCD layer, into the material below the PCD layer until the crack length is sufficient for spalling, chipping, or catastrophic failure of the enhanced insert.
The impact, wear and fatigue life of the diamond layer may be increased by increasing the diamond thickness and thus, the diamond volume. However, the increase in diamond volume results in an increase in the magnitude of residual stresses formed on the diamond/substrate interface which foster delamination. This increase in the magnitude of the residual stresses is believed to be caused by the difference in the thermal contractions of the diamond and the carbide substrate during cool-down after the sintering process. During cool-down after the diamond bonds to the substrate, the diamond contracts a smaller amount than the carbide substrate resulting in residual stresses on the diamond/substrate interface. The residual stresses are proportional to the volume of diamond in relation to the volume of the substrate.
Both the fatigue and impact failure mechanisms involve the development and propagation of Hertzian ring cracks which develop around at least part of the periphery 1279 of the contact area 1280 with the earth formation (FIG. 12B). This part of the periphery of the contact area is referred to herein as the "critical contact region" of the insert and is denoted by reference numeral 1279 in FIG. 12B. These ring cracks which develop in the critical contact region typically propagate in a stable manner through the ultra hard material layer in a direction away from the contact region. Microscopic examination of inserts which have been used in drilling applications show that it is not the development of surface cracks in the PCD which limits the useful life of the cutting element, but rather the impact or fatigue induced propagation of these surface cracks into the substrate material which limits the useful life of the inserts.
There is, therefore, a need for an insert with increased resistance to the localized wear, fatigue and impact resistance mechanisms so as to have an enhanced operating life. To solve this need, the inserts of the present invention have an increased thickness of diamond in the critical contact region.
In efforts to increase insert cutting life, applicants discovered that it is advantageous to place thicker PCD in the critical contact region and in areas immediately outside the contact area where fatigue or impact induced crack growth is of primary concern. In practical drilling applications, the critical contact region can vary substantially due to the intrinsic variations in depth of contact with the earth formation during drilling. These variations in the depth of contact may be due to, for example, the inhomogeneity in the formation, and the weight on the bit. Because of this variation, it was found necessary to place the thicker PCD in a certain defined region rather than at a single location. This defined region includes the critical contact region and is referred to herein for descriptive purposes as the "critical zone." Moreover, by limiting the thicker diamond to a defined region, the increase in the volume of the diamond is minimized, therefore minimizing the increase in residual stresses.
The prior art does not disclose such an insert. For example, U.S. Pat. Nos. 5,379,854 and 5,544,713 disclose inserts having a corrugated interface between the diamond and the carbide support. These corrugated interfaces create a step wise transition between the two materials which serves as structural reinforcement for the transfer of shear stress from diamond to the carbide and thus, reducing the amount of the shear stress which is placed on the bond line between the diamond and the carbide. Moreover, the corrugated interface reduces the thermally induced stresses on the interface of the diamond and carbide due to the mismatch in the coefficient of thermal expansion between the two materials.
To increase the resistance to cracking, chipping and wear of the diamond layer of the insert, U.S. Pat. No. 5,335,738, discloses an insert having a carbide body having a core containing eta-phase surrounded by a surface zone free of eta-phase. It is believed that this multi-structure insert body causes a favorable distribution of the stresses created by the coefficient of thermal expansion mismatch between the diamond and the carbide. Moreover, the '738 patent discloses depressions on the protrusion of the insert body beneath the diamond layer. These depressions are filled with diamond material different than the diamond material which makes up the diamond layer in cutting elements.
Neither of the '854, '713, or '738 patents teach a way of overcoming the localized failure modes nor do they teach the placement of an increased thickness of diamond on the area of contact between the diamond and the earth formation.